In the present invention, unauthorized access from outside a facility to a device disposed inside the facility is detected by effectively using the output from a mirror port of a network switch. A gateway device has: a monitored data acquisition unit for saving in a monitored data storage unit, as monitored data, packet data that is outputted from a mirror port of a switch, the packet data being outputted from a device being monitored; an unauthorized access detection unit for detecting unauthorized access by determining whether the monitored data is abnormal on the basis of a comparison between the monitored data and assessment rules; and an unauthorized access notification unit for notifying a server of a monitoring center, which is connected to an external network via an external communication unit, that unauthorized access has been detected.